Recovery of file system data in file servers mirrored file system volumes

Abstract

The invention provides a method and system for recovery of file system data in file servers having mirrored file system volumes. The invention makes use of a “snapshot” feature of a robust file system (the “WAFL File System”) disclosed in the Incorporated Disclosures, to rapidly determined which of two or more mirrored volumes is most up-to-date, and which file blocks of the most recent mirrored volume have been changed from each one of the mirrored file systems. In a preferred embodiment, among a plurality of mirrored volumes, the invention rapidly determines which is the most up-to-date by examining a consistency point number maintained by the WAFL File System at each mirrored volume. The invention rapidly pairwise determines what blocks are shared between that most up-to-date mirrored volume and each other mirrored volume, in response to a snapshot of the file system maintained at each mirrored volume and are stored in common pairwise between each mirrored volume and the most up-to-date mirrored volume. The invention re synchronizes only those blocks that have been changed between the common snapshot and the most up-to-date snapshot.

Description

[0001]This application hereby incorporates by reference and is a continuation of U.S. application Ser. No. 09 / 684,487, filed Oct. 4, 2000, now U.S. Pat. No. 6,654,912.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The invention relates to recovery of file system data in file servers having mirrored file system volumes.[0004]2. Related Art[0005]Network file servers and other file systems are subject to errors and other failures, including those arising from hardware failure, software error, or erroneous configuration. Because of the possibility of error, many file systems provide additional copies of data in the file system, such as by providing a mirrored file system volume. In a mirrored file system, a first volume provides a first copy of the file system, while a second volume provides a synchronous, second copy of the file system. Thus, if data on the first volume is corrupted or otherwise lost, data from the second volume can be used in its place transparently.[...

AI Technical Summary

Benefits of technology

"The invention is a method and system for quickly and accurately recovering data from file servers with mirrored file systems. It uses a consistency point model and a snapshot feature of a robust file system to determine which mirrored volume has the most up-to-date data. The invention then compares the data between the most up-to-date mirrored volume and each other mirrored volume, and only copies those blocks that have been changed. This allows for a faster and more reliable recovery of data from redundant file systems. The invention has a wide range of applications, including file servers with mirrored file system volumes and RAID subsystems."

Problems solved by technology

Network file servers and other file systems are subject to errors and other failures, including those arising from hardware failure, software error, or erroneous configuration.

One problem in the known art is that the first volume and second volume of the file system can fail to remain in synchronization.

As a result, the first volume and second volume will no longer serve as accurate mirrors for each other because one is out-of-date.

An aspect of this problem is that, after system crashes, it is unknown which of the first volume and second volume is the most recent.

While this method can generally achieve the result of assuring that the first copy and second copy of the file system are synchronized after system crashes, it has the severe drawback that it is very inefficient, as each file block of at least one of the mirror file systems must be copied to the other one of the mirror file systems.

When the file system is particularly large, such as one that approaches or exceeds a terabyte in size, this drawback makes this known method untenable due to its incredible latency (and liability to other failures).

While this method can generally achieve the result otherwise achieved by the first known method, is still subject to at least two drawbacks.

First, this method is more complex, in that it requires careful maintenance so as to ensure that the log remains synchronous.

Second, the log itself must generally be mirrored for reliability, which of course re introduces the entire problem of recovery of mirrored files after system crashes.

Third, maintaining this additional log increases the latency of every operation.

Moreover, such a technique can introduce additional errors in the event that the log is unreliable.

Images